When AC power supply voltage is rectified by a full wave rectifier circuit, the input current is distorted and the power factor drops. Therefore in a conventional well-known power factor correction circuit, a boost chopper, constituted by an inductor (boost reactor), a semiconductor switch, a diode for rectification and a smoothing capacitor, is connected to the output side of the full wave rectifier circuit, and the distortion of the input current is suppressed by the switching operation of the boost chopper.
FIG. 8 is a power factor correction circuit of this kind disclosed in Japanese Patent No. 4363067 (e.g., ¶¶[0026] to [0052] and FIG. 1 to FIG. 8 thereof).
In FIG. 8, 10 is an AC power supply, 20 is a full wave rectifier circuit constituted by a diode bridge, 31 is an inductor, 32 is a current detection resistor, 33 is a semiconductor switch, 34 is a diode for rectification, 35 is a smoothing capacitor, and 40 is a load. Here the inductor 31, the semiconductor switch 33, the diode for rectification 34 and the smoothing capacitor 35 constitute the boost chopper, which repeats storing and releasing energy to/from the inductor 31 by turning the semiconductor switch 33 ON/OFF, so as to boost the voltage of the smoothing capacitor 35 to a DC voltage that is higher than the output voltage of the full wave rectifier circuit 20, and supply the boosted voltage to the load 40.
50 is a control circuit that controls the semiconductor switch 33, 51 is an error amplifier that amplifies an error between a reference voltage 52 and an output voltage of a main circuit, 53 is a multiplier that multiplies the output of the error amplifier 54 by the positive side terminal voltage of the full wave rectifier circuit 20, 54 is an error amplifier that amplifies an error between the output of the multiplier 53 and the negative side terminal voltage (voltage at one end of the current detection resistor 32) of the full wave rectifier circuit 20, 55 is a voltage control oscillator (VCO) that outputs a triangular wave signal in accordance with the magnitude of the positive side terminal voltage of the full wave rectifier circuit 20, and 56 is a PWM comparator that compares a feedback signal FB output from the error amplifier 54 and the triangular wave signal output from the VCO 55, and the semiconductor switch 33 is driven by a PWM pulse output from the comparator 56.
In this prior art, the PWM comparator 56 generates the PWM pulse that continuously compensates the fluctuations of the AC power supply voltage and the DC output voltage, and the input power factor is corrected by matching the AC current waveform with the AC voltage waveform.
At the same time, as shown in FIG. 9, switching frequency f of the semiconductor switch 33 is changed by the function of the VCO 55 in proportion to the magnitude of the AC power supply voltage Vin, whereby normal mode noise generated by switching is dispersed with respect to the frequency, and noise and switching loss are reduced.
In terms of the operation of the circuit shown in FIG. 8, the error amplifier 51 amplifies the error between the reference voltage 52 and the DC output voltage of the main circuit, and the result of multiplying this error voltage by the positive side terminal voltage of the full wave rectifier circuit 20 is input to a non-inverting input terminal of the error amplifier 54. The error amplifier 54 computes error voltage based on this multiplication result and voltage at one end of the current detection resistor 32, and this error voltage is input to a non-inverting input terminal of the PWM comparator 56 as a feedback signal FB.
On the other hand, the VCO 55 has a voltage frequency conversion characteristic to change the frequency f from f1 to f2 in a range from the lower limit value E1 to the upper limit value E2 of the input voltage E, as shown in FIG. 10, and outputs a triangular wave signal of which upper limit frequency is f2 shown in the upper level of FIG. 11A, or a triangular wave signal of which lower limit frequency is f1 shown in the upper level of FIG. 11B, for example, in accordance with the magnitude of the input voltage E.
The comparator 56 compares the triangular wave signal and the feedback signal FB output from the error amplifier 54 and generates the PWM pulse shown in the lower level of FIG. 11A or FIG. 11B, and switches the semiconductor switch 33 by this PWM pulse.
According to the prior art disclosed in Japanese Patent No. 4363067, noise is suppressed by dropping the switching frequency f to reduce the switching loss in a range where the AC power supply voltage Vin is particularly low, and dispersing noise due to switching with respect to the frequency, as mentioned above.
However this prior art is based on the concept that the switching frequency f is simply changed in proportion to the magnitude of the AC power supply voltage Vin. Therefore depending on the magnitude of the AC power supply voltage Vin, the noise reduction effect may not be demonstrated sufficiently, and a large filter circuit, including a noise mode coil or the like, must be additionally used in order to satisfy the predetermined conduction noise regulation.